It would be a big deal for someone to only come out as nonbinary when they're old enough to be an ensign on a starship. How many times in high school did they say "No, I'm not nonbinary, I'm just a girl"? How many times did they insist to their parents on getting a girly thing because they wanted to fit in with a feminine friend group?

Social anxiety is always a valid emotion, even if you know the society is progressive enough that it isn't really a big deal.